Cost to run Fridge/Freezer - Propane v AC

I was wondering how much cost difference there is between operating the Fridge/Freezer on Propane v AC

So, I contacted Dometic who came back with the following info "The unit uses approximately 2200-2400 BTU's per hour and 3.8 volts ac per hour. "

Please feel free to tell me if my calculations are wrong:-

Running on LPG

1 Litre of Propane equals 24,170 BTU

If we asume an average of 2300 BTU per hour, it gives 55,200 BTU in a day or 2.28 litres of Propane per day.

I am currently paying ?0.37 per litre so running my fridge/freezer on propane will cost me just ?0.84 per day (or ?5.91 per week).

Running on AC

Now, I dont know how to calculate "volts ac per hour" but I do know my unit uses 660W.

Taking a unit of electricity (which is 1000 watts of power used for 1 hour) at ?0.17 per unit, the calculation is as follows ; ?0.17 x 660 / 1000 = ?0.11 per hour or ?2.69 per day (making ?18.85 per week).

Conclusion

If you are on a site with electric included, enjoy a nice beer in the knowledge that the campground is spending nearly ?20 a week to keep it cold for you.

If you are on a meter, turn the fridge/freezer to LPG NOW !!!!!!

My figures above are in GBP and the price you pay for propane/electric will be different, but you can do your own calculations too

Paul

PS I am now trying to work out the cost of using electric heating v propane so does anyone know what BTU per hour a std furnace uses (and watts for the electric pump).

Hi thanks for the info. Just a couple of notes; it wouldn't be "volts per hour" it is amps. Also: the electric is pretty much (for arguments sake, and the small scale involved here) 100% efficient, but the propane is more in the area of 80%. So 20% less efficiency should be added to the propane cost.

it wouldn't be "volts per hour" it is amps.

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Would that give a different calculation and/or result to what's above?

the propane is more in the area of 80%. So 20% less efficiency should be added to the propane cost.

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Good point - I forgot that one.

Cheers
Paul

RVOA said:

I was wondering how much cost difference there is between operating the Fridge/Freezer on Propane v AC

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Paul,

We have an article in our library which can be use to calculate the relative cost of operating on propane or AC. A direct link is found at Propane vs Electric. I am not sure about the currency and/or KWH conversions for your country. Do you use KWH or some other unit of electricity usage? The difference between 50 and 60 Hertz might also be a factor.

I hope this helps.

Hi Richard

Thanks for that link (note to myself: check the library  :)

The program obviously calculates at US gallons where UK gallons are different, so I would need to adjust for that.

Obviously the $ and ? symbols are different but I can just imagine the ? symbol for that one.

Does the usage of the appliance not change dependant on each appliance as there doesnt seem to be any way to change that?

Thanks
Paul

Another thing to consider is how the power consumption is listed.  Is it 660W while the compressor is running or is that the average power use?  It seems like they would have a hard time listing the average power consumption because that would depend on many factor such as the outside temp and how often the fridge is opened.  If it's 660W while the compressor is running then I think your cost for AC woud go down dramatically.

If it's 660W while the compressor is running then I think your cost for AC woud go down dramatically.

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Would the LPG cost reduce too or is the compressor just used on AC (sorry, maybe stupid question but I dont know).

Paul

RV refrigerators don't have a compressor.  The electricity is used to run a heater so frequency should have little or now effect on the power consumption as it's a resistive load.

RVOA,

I don't think you have enough information to complete the analysis you are attempting to complete.

First of all, electrical loads are measured in amps, not volts, since your AC voltage will remain (in theory) constant when plugged in.  If the tech meant to say 3.8 Amps or Amp-hours, then we could complete the math.  IIRC, UK operates on 240 volt system (50hz doesn't matter when calculating power draw).  3.8 Amps at 240 volts is 912 watts, which for an hour would be 912 watt-hours or .912 kWh (kiloWatt-hours). 

Btus are just another form of energy measurement, so if we convert kWh to Btus we find that the electrical heater is using 3112 Btus per hour (1 kWh = 3412 Btus), compared to the estimated 2300 Btus per hour on propane.  As mentioned, the propane will yield about 80% of its heat in a refer application (minimum 20% heat loss), which would net out to 1840 Btus per hour used by the absorption chiller.  Obviously, something doesn't quite add up between the propane and electric values you were provided by the manufacturer, since the energy actually used by the unit should be equivalent.

Second, I believe the tech may have provided you with capacity numbers.  In other words, how much "can" the refrigerator use in an hour.  The actual usage will be dictated by many factors including external temperature, internal rig temperature, refrigerator cold setting, how much warm food you just put in it, etc.  I suspect your actual usage will be much less than the values provided by the manufacturer. 

Think of it this way...if the estimates provided to you were accurate, you would be losing the amount of heat equivalent to running a 900 watt heater. 

Another "gut check" comparison, would be to consider a household refrigerator (different design and thermal cycle I know), but an average residential style refrigerator will use 700 kWH per year.  If your RV unit were to run all year at the suggested wattage given to you, it would use 7,989 kWh per year.  Again, something seems amiss in the numbers you were given.

Assuming the values were correct, the analysis would be as follows:

Btus Gas = 2300 / 24,170Btu per L x 0.37quid x 8,760 hours per year =  308.43 quid/year
kWh Electric = .912 kW per hour x .17 quid per kWh x 8760 hours per year = 1358.15 quid/year

The reason electricity is typically more expensive on a Btu basis is that you lose approximately 65% of the energy in fuel when it is converted to electrical energy.  This means, to make heat, burning fuel at the source is MUCH more efficient than burning fuel to make electricity and using that electricity to make heat.  It's not that simple of course, but you get the gist.

Regards,
Mc2guy